In recent years, minimally-invasive surgical techniques have become the preferred methods of performing many procedures that were previously carried out through an open incision. The adoption of these minimally-invasive techniques has gone hand-in-hand with the development of methods of visualizing the position of a surgical tool being manipulated inside the body. Although endoscopes offer a preferred mode of visualization in some areas of surgery, they are unsuitable for use in many procedures, such as neurosurgical and orthopedic procedures, in which tools must be inserted and manipulated very delicately in narrow spaces with poor optical visibility. In spinal surgery, for example, and in particular, in treatments of the intervertebral discs, a thin, hollow needle must be inserted near the center of the intervertebral space, in such a manner as to aspirate the fluid disc matter without touching the spinal cord, spinal nerves and blood vessels nearby.
In neurosurgery, before performing surgery, a three-dimensional image of the patient's head is formed, preferably using a CT imaging system. The image is used by the surgeon, as is known in the art, in planning the procedure and, preferably, in establishing a three-dimensional frame of reference for the operation, fixed with respect to the patient's anatomy. During the surgery itself, as the surgeon inserts and manipulates a surgical tool, its position is tracked in relation to the frame of reference. A stereotactic frame may be fastened to the patient's skin or bones, to be used in tracking and guiding the position of the needle.
Various methods are known in the art for tracking the position of a surgical tool with respect to the anatomy of a patient. For example, Medivision Advanced Support Systems, of Oberdorf, Switzerland, offers a system for spinal surgery that includes an optical position sensor fixed to a surgical tool and a reference element having three optical fiducial marks, in a fixed, predetermined spatial relationship. The reference element is fixed to the patient's back in a known position, and a camera is used to track the movement of the tool, relative to the reference element.
PCT patent publication WO 96/08209, whose disclosure is incorporated herein by reference, describes a combined position tracking and imaging system for use in medical applications, using a reference frame secured to a patient's head. The system monitors the position of a surgical instrument relative to the reference frame, using a mobile sensor, such as electromagnetic field sensor, as is known in the art, fixed to the instrument. Prerecorded images of the patient's body, generally CT images of the patient's head, are displayed responsive to the monitored position of the instrument relative to the body. The position of the instrument is registered on the prerecorded images.
Preferably, before the surgery, the frame is fixed to the patient's head, and a set of CT images is acquired. These images are used to register the position coordinates of the frame, including the coordinates of a reference position sensor therein, in relation to the patient's anatomy. Subsequently, during the surgery, signals output by the reference and mobile position sensors are monitored so as to track the coordinates of the sensors. The coordinates of the mobile sensor relative to the reference are used to register the position of the instrument with respect to the patient's anatomy, for example using the previously-acquired CT images.
Similarly, U.S. Pat. No. 5,383,454, whose disclosure is incorporated herein by reference, describes a position tracking and imaging system for use in neurosurgery. Before surgery, ultrasonic emitters are fixed to a number of reference points on the patient's head, and a set of CT images of the head are produced showing the positions of the reference points. Similar emitters are fixed to a surgical probe for insertion into the head. During surgery, an array of microphones in the operating room receive ultrasound signals emitted by the emitters on the patient's head and on the probe. These signals are used to determine the position and orientation of the probe relative to the reference points. The position and orientation information is used to display an image of the probe superimposed on the prerecorded CT images.
Position determination procedures are also described, for example, in U.S. Pat. Nos. 5,558,091, 5,391,199, 5,443,489 and 5,377,678, all of which are incorporated herein by reference. Position determining systems -generally use extrabody apparatus to locate a sensor attached to the surgical tool. The extrabody apparatus includes one or more field transducers, generally radiators or receivers, positioned above and/or around the patient, which transmit fields to and/or receive fields from the sensor. Each radiator or receiver has a characteristic "detection volume" in which the fields have sufficient strength in order to generate a strong enough signal in conjunction with the sensor, such that the location of the surgical tool can be determined to a desired level of accuracy.
The size of the detection volume is generally dependent on the size of the radiators or receivers. In some types of surgery, such as back surgery, the size of the detection volume may cause limitations on the surgery. If large radiators are used, they may interfere with the movements of a physician or other medical-staff member. Small radiators, which do not occupy much space, may not have a large enough detection volume and/or may have low resolution.
Although the position sensing system may be used to register the position of the tool with previously-acquired CT or MRI images, as described above, surgeons are generally unwilling to rely only on prerecorded images. In addition to the use of a reference frame or reference points and position sensors to track a surgical tool, as described in the above-mentioned PCT publication and in U.S. Pat. 5,383,454, for example, fluoroscopic X-ray imaging is generally also used to verify that the tool is indeed at the position indicated by the position sensors. This verification is needed, inter alia, to ensure that the frame has not shifted relative to the patient's anatomy, and that the position readings from the position sensors have not drifted. An error in the angle and depth of penetration of the tool can, clearly, have devastating consequences.
Typically, two-plane fluoroscopic X-ray imaging is used, wherein two perpendicular X-ray images are formed simultaneously, one an anterior-posterior image (top to bottom) and the other a lateral image (side to side). The two-plane fluoroscope is costly, however, and must be operated substantially continuously to monitor the position of the surgical tool resulting in undesirably high radiation dosages to the patient, as well as to the operating room staff. Furthermore, the fluoroscopic images acquired during surgery are not registered with the previously-acquired CT images or with the coordinates of the reference position sensor, so that there is no convenient way to re-calibrate the readings of the position sensors if they are found to be erroneous.
U.S. Pat. Nos. 5,265,610 and 5,577,502 suggest performing invasive medical procedures during which multiple X-ray images are periodically acquired, to give the operator information on the three-dimensional location of an invasive tool. In order to minimize the X-ray dosage to the patient, RF transmitters and receivers are used to receive positional information on the invasive tool. The positional information from the RF transmitters is used to superimpose the position of the tool on the X-ray images. The patient's motion may also be tracked, and the image display adjusted accordingly. Thus, it is maintained that the X-ray images may be updated less frequently than in conventional X-ray tracking systems.